The FlashAvatar Crossover
by heyitsthatgurl
Summary: A black hole from the Flash dimension opens in the Avatarverse and Aang and Katara get sucked in! They meet the Flash and his friends, together they figure out how to get Aang and Katara back home. Rated T just because that's just what I do. Picks up after the season 1, and about three years after the series finale of Avatar. (The whole gang is there in both universes)
1. Chapter 1

**Hello! So I made a poll on whether I should make this or not, and I decided to do it anyway! The idea popped into my head at a random moment and I started typing it later. I hope you like it!**

 **Avatarverse:**

The Gaang flew along the coast of Ember Island, it was quiet. They were on their way to a diplomatic meeting in Yu Dao about the new city that was planning to be built, but they were stopping in Ember Island to pick up Zuko and Mai and for a much needed couple days of rest. The sun was setting and after hours of flying they were getting ready to land. Sokka, Suki and Toph were asleep but Aang and Katara sat on Appa's head. Aang guided Appa to where they needed to go, Katara leaned her head on her boyfriend's shoulder.

This had turned into one of Katara's favorite places, with her family, friends and boyfriend. It was so peaceful, it was a good place to think. She thought about the future, the past and many other things that crossed her mind.

Aang and Katara had been dating for a while now, almost four years, and now that she was sixteen, she was expected to find a husband. Aang was fifteen which was an adult by his culture's standards, he wanted to honor the Southern Water Tribe's customs so they agreed to wait another year.

"Hey Aang are we there yet?" asked a still half asleep Sokka

.

"Not yet, but you should wake up Suki and Toph." Aang replied

.

"I'm awake Twinkletoes, so is Girly over here." Toph said.

It wasn't long before they started descending from the clouds into the fading daylight on Ember Island.

Aang noticed a strange force in the distance, a hole in the sky. Instincts kicked in and he flew a little closer to it, not to close but close enough to get a good look at what was going on.

"What's that thing?" Katara asked taking her head off of his shoulder. Sokka and Suki moved over to get a better view, and they sat with their mouths open.

The hole in the sky seemed to be growing, it got bigger and more powerful. It suked up anything below or around it. Strong waves of turbulence approached them, knocking them all out of balance.

"Hold on to something!" Aang yelled.

Everyone did what he said, Sokka and Suki grabbed on to each other and Appa's saddle, Toph clung to the side of the saddle and Katara clung onto Aang while Aang clutched the reins. The wind kept getting stronger, Appa was barely fighting the wind. Katara hung on tighter, she could feel the wind sucking her in, making her surrender at it's mercy. She hung on tighter still, but it wasn't good enough.

The strong winds yanked her from her family, taking her into the inevitable, was she to live or die she wasn't sure. She screamed as she was pulled further and further away from her family, her friends and Aang.

"KATARA!" Aang yelled. He grabbed his staff ready to go catch her. But a hand stopped him before he could stand up.

"Where are you going?!" Sokka asked in a panicked state.

"Sokka, you have to steer Appa away from here, we'll catch up with you. And if anything happens do not follow us." Aang said in an orderly tone.

Sokka noded as he took the reins, Aang jumped off of Appa with his glider in hand to save his beloved.

Katara was still being pulled by the hole in the sky, closer and closer. Never had she been so scared in her life, other times she had been scared she would know the outcome of whatever what was happening but here, she had no idea what would happen.

Aang was getting closer and closer to Katara, he was flying as fast as he could. He was too late, just as he was about to reach her she fell into the hole, the only thing he could do was follow her, wherever she went.

Aang dove head first into the hole, praying to the spirits that he wasn't too late.

Sokka and Suki watched as the circle closed just after Aang entered it.

 **Central City:**

Barry was running circles around the opposite direction of the singularity, it seemed to be letting up, slowing down until after another fifteen minutes of running at his top speed it stopped all together. Right before it closed though, a figure was spit out into the sky, it sounded like a screaming girl, a helpless screaming girl that would fall to her death if he didn't catch her.

"Cisco a girl just came out of the black hole. She's falling out of the sky!" Barry said into the communicator.

"Well the black hole seems to be closing. I think it's safe for you to run down and catch her." Cisco replied.

Barry watched the hole close, then ran as fast as he could to the girl who had suddenly gone limp.

 _Great, now she's probably unconscious_ Barry thought as he ran down a building. He ran as fast as he could, luckily the wind was pushing her in his direction, right before she hit the building that he was running down he caught her.

Before he completely ran out of steam he managed to muster up enough energy to get back to Star Labs, he placed the girl on the hospital bed and fell onto the floor out of pure exhaustion.

"Barry are you OK?" Iris asked Barry, rushing to his side to help him up.

"Yeah, I'm fine but I don't think that she is" he replied.

 **There you have it! Sorry I left you with a cliffhanger but I wanted to keep people interested so yeah. Please review, I would love to hear your feedback!**


	2. Chapter 2

**Hey! So this is how it's going to work, most likely this will be updated daily because I have been writing everything as one big story then splitting it into chapters. Now that it's summer vacation I will have more time to update and write things.**

 **Disclaimer: I do not own The Flash or Avatar the Last Airbender.**

Everyone looked at the girl, she had tanned skin and long dark brown hair. She was wearing a blue dress that went to her knees, with leggings and dark brown boots. She was also wearing wraps on her hands that stopped just before her elbow, on her neck she was wearing a necklace that had a gem with a strange foreign symbol carved into it.

"Uugh, what happened?" the girl moaned. She opened her eyes, her ocean blue eyes. Once she realized where she was they bulged wide and she looked scared and confused.

Barry was the first person to speak.

"Hello" he said nicely

.

" Where am I?" the girl asked.

"You're in Central City." Barry replied.

"I've never heard of it" she said looking around.

"Where are you from?" Caitlyn asked.

"The Southern Water Tribe" replied the girl.

"Can you tell us how old you are and what your name is?" Joe asked from the back.

"My name is Katara and I am sixteen years old. I am a Waterbending Master and a Master Healer" Katara said.

"Does that mean that you are a kung fu ninja master or something?" Cisco asked excited.

"You don't know what bending is? At all?" Katara asked, she seemed surprised as if it was common knowledge. They all shook their heads no. She tilted her head with a puzzled look, her eyes darted around the room, they stayed focused on the cup on the table. She stood up and lifted her arms. In one graceful movement she swirled her hands around, the water in the cup started to rise and go to her. She made it twirl and freeze. with the simplest movements.

"That is called water bending, and this is healing" Katara said as she approached Barry. She moved her hand with the water in it to his cuts from closing the black hole, the water started to glow and in less than a minute they were gone.

"Wow, that's so… AWESOME! What else can you do?" Cisco asked, earning a slap on the arm from Caitlin.

"I can fight with my waterbending. That's it. But that was the fastest I've ever healed anyone." Katara said, examining Barry.

"Oh, I heal fast anyways. I'm pretty fast at everything" He said with a smile.

"How fast exactly?" she asked. Barry smiled wider and flashed to her, picked her up and took her to the outside of Star Labs. He put her down for a minute and she stared at him with her eyes wide in awe. Barry picked her back up and flashed her back to the Coretex.

"That was amazing! Not even Aang can run that fast and he's an airbender!" Katara said.

"Who's Aang?" Iris asked. Katara's face fell.

"He's my boyfriend." she replied sadly.

"Did you two break up?" Iris asked her sympathetically.

"What? No, no. I just don't know if I'm going to see him again. I don't even know how I got here!" she said frustrated, the water started to shake a little as if it shared her emotion. Iris comforted her as Barry turned to Ronnie and Caitlin.

"So, Barry, she came from another universe right?" Ronnie asked. Barry nodded.

"Then how are we going to get her back?" Ronnie asked again. Barry looked at Dr. Stein and he put his hands in the air.

"Don't look at me! I've never had any experience with any of this I had theories" Dr. Stein protested.

"Well, she's going to be here for a while. We need her to blend in as much as possible." Caitlin.

"How are we going to do that?" Joe asked with a "I don't know about this" tone.

"I think I know someone who can help us. " Barry said as he flashed away.

 **Please leave a review!**


	3. Chapter 3

**Hello! Here is chapter three. Please leave a review!**

 **I do not own the Flash or Avatar.**

Katara was in tears. She had realized that she was in a completly different universe and there was probably no way back.

"Hey calm down it's Ok." Iris said. It took a second for Katara to calm down. She took a deep breath and stood up. She looked at Caitlin.

"What do I need to do to get home?" She asked determined. Everyone looked at her with a sympathetic face.

 **Avatarverse:**

"Calm down Snoozles. I'm sure that they are fine" Toph said, fed up. Sokka had been pacing up and down the same patch of grass for almost an hour, his heartbeat was going faster than Aang and Katara's when they were kissing.

"Sokka the best thing we can do for them right now is wait." Suki said, even though she couldn't feel his heartbeat she too was sick of him being in this panicked state.

"NO THERE HAS TO BE SOMETHING WE CAN DO!" He yelled at both of them. Suki's eyes bulged a little and Toph got mad.

"ALRIGHT THEN SNOOZLES! WHAT DO YOU SUGGEST?" Toph yelled at him.

"Ok, I know you're both mad but we have to stop fighting for Aang and Katara!" Suki butted in. "Maybe there's someone who can help us. I think that we should go pick up Zuko and Mai like we were going to, maybe they know someone that can help us."

Suki took Sokka and Toph and made them get on to Appa. Sokka and Toph sat on the opposite side of the saddle and Suki steered Appa.

 **Starling City:**

Barry flashed into the Arrow cave and was surprised to see that there wasn't anyone there. He flashed to the Top of Palmer Tech, everyone from the Arrow cave was there, except for Oliver and Felicity. Diggle jumped when he got there and Thea looked at him in awe (she had never seen the Flash before).

"Hey Diggle. Where's Felicity? I need her help for something important." he said

"Felicity and Oliver are on leave. What do you need help with?" he replied.

"I need her to make a fake ID for a girl that fell from the black hole, we need her to blend in until we can find a way back for her to go home." Barry said.

"Ok, then why don't you call her?" Thea spoke up. Barry looked at her then at Diggle.

"She's Oliver's sister." Diggle said. Barry smiled at her then flashed to her.

"Nice to meet you, I'm Barry Allen." Barry said. He flashed to the phone and dialed Felicity's number.

"Hello?" she spoke through the phone.

"Where are you?" Barry asked

"Barry? I"m in Coast City with Oliver" Felicity replied confused. Barry dropped the phone and flashed out of Palmer Tech. He ran to Coast City and went to all the places he thought they might be. Finally he found them walking along the bridge. He flashed Felicity off the bridge to an alleyway then did the same with Oliver.

"Barry, what are you doing here?" Oliver asked him slightly angry.

"Felicity I need a favor" he said. turning to Felicity.

 **Central City:**

Everyone was talking to Katara, Dr Stein was about to go home and see his wife when the alarm sounded, like it did when the black hole opened again.

"What's going on?" Katara asked.

Dr Stein ran back to the computer and looked alarms stopped after a minute. He looked up and said.

"I think we should go outside. A portal just opened for a second, I think something came out of it." Dr Stein said.

"How do you know that?" Joe asked Dr Stein, confused.

"I'm getting the same reading of when Katara fell out of the black hole on a smaller scale, because it just opened for a second the black hole is a minor threat. This smaller hole that opened does not have the energy to become a singularity." Dr Stein replied. Joe looked at him as if he was confused. "It means Barry isn't needed." Dr Stein said annoyed. Joe nodded his head.

Everyone headed outside and Katara followed them, when they got out they all looked up at the sky, sure enough there was a figure in the sky but it wasn't falling like Katara was, it was gliding along the sky, slowly coming down. Katara inhaled excitedly, as if she knew what it was.

"What? Do you know what that is?" Cisco asked her.

"It's Aang!" she said exitedly.

 **Please review!**


	4. Chapter 4

**Hello! Sorry I haven't posted in a while, I promise I will be more active.**

 **Disclimer: I do not own the Flash or Avatar.**

 **Coast City:**

"It won't take that long I promise." Barry said. Felicity looked at Oliver with a look that said "please".

"Fine, we were planning on going to Central City anyway. I'll meet you there and bring your stuff." Oliver said with a sigh.

"Thank you" Felicity said. She leaned to kiss him but Barry flashed her back to Central City.

Central City:

Aang landed close to Star Labs, once he landed he looked around frantically, like he was scared.

"Aang!" Katara yelled. He turned to look at her, when he saw her his body relaxed. They ran to each other and shared a hug. Then they looked at each other and kissed. Once they pulled away they hugged again.

"I thought I lost you. Are you OK?" Aang asked her.

"I'm fine, I fainted on the way down but these people saved me." Katara replied.

Aang and Katara stopped hugging but still held hands. Katara introduced Aang to everyone and they walked back to Star Labs. When they got there Barry was standing next to Felicity.

"Katara this is Felicity, she's going to make you a fake ID and make it seem like you belong in this universe. Um, who is this" Barry asked looking at Aang.

"Oh, this is Aang he's my boyfriend. He uh, just got here." Katara replied to Barry.

"Hi. I should be the last person to uh, fall from the sky." Aang said.

"Are Sokka, Suki and Toph alright? What happened to them?" Katara asked him suddenly slightly panic stricken.

"They're fine Katara, I sort of ordered Sokka to land Appa and not to follow us, in the tone I use in meetings." Aang replied rubbing the back of his neck.

"Ok, that's fine. He must be worried sick right now." Katara said sighing.

Everyone just watched as they had their own conversation. They kept babbling on about "Sokka" and "Appa" and various other things that no one could understand. Finally after a few minutes Joe coughed and they stopped looking embarrassed.

"Sorry." They said.

"It's ok. Let's make you guys 'real' ". Felicity said. She walked over to the computers and tapped into the records of births and deaths from around the country.

"First we need new names for you both, not that there is anything wrong with your names it's just that, they're um, very exotic and might draw attention to yourselves." Felicity said in her awkward but cute way. Barry laughed a little.

"Well what about Gyatso? Or Pakku?" Aang threw out there. Felicity shook her head.

"Is Kya alright? or Kanna?" Katara asked hopefully. Felicity shook her head again.

"How about we just pick new names for you." Caitlin said smiling.

"Oh! I know, for Katara we'll do Katrina but we'll shorten it to Kat. That's easy to remember." Iris said.

" I like that. Is that Ok Katara?" Felicity asked, Katara nodded. Felicity typed on the keyboard.

"What should we do for Aang?" Felicity asked.

"Anything is fine, I don't mind." Aang said shrugging.

"Ok, so your names are Kat Clayton and Adam Branson". Felicity said.

"Branson?" Barry asked sarcastically.

"Do you have any other ideas?" Felicity asked.

"How about Smith? It's better than Branson." Barry replied.

"Fine, Smith. I can do the rest by myself" Felicity said, pouting slightly.

Avatarverse:

Suki, Sokka and Toph traveled to Ember Island where they met Zuko and Mai as planned. They were surprised to see that Aang and Katara weren't with them, they were even more surprised when the heard what happened. Zuko sat with his mouth open for a minute and Mai even raised her eyebrows.

"So do you know ho can help us?" Suki asked them.

"Well, this hole in the sky sounds like it might be the spirits. We could try talking to Uncle, he might know." Zuko replied, still in shock.

"Where is he?" Sokka asked him.

"Probably in Ba Sing Se tending to his tea shop. We should be able to fly there in a couple days." Zuko said.

"We can't waste any time! Pack your stuff we're leaving in an hour!" Sokka said.

"Sokka, Appa needs rest, he's been flying all day." Suki pointed out. Sokka opened his mouth to argue but he knew she was right.

"Fine, but we leave first thing in the morning." He replied.

There was awkward silence for a while afterwards.


	5. Chapter 5

**Hello! Merry Christmas! I am so so so so sorry that I haven't updated in a long time. I've been really busy with school and exams so I haven't had time to write. I'm sorry this is short too. Hopefully I'll post the second half of this tomorrow ;)**

 **I don't own Avtar the last Airbender or the Flash**

 **Central City:**

It took about two minutes for Felicity to finish.

"Alright, Kat." She said to Katara "If anyone asks about your name, say it's short for Katrina." Katara nodded.

"And Adam, you need a hat or something. What are we going to do about that giant arrow on your forehead? Actually, what are we going to do about your appearances in general?" Felicity asked everyone.

Aang and Katara looked at each other. They looked at their clothes, then at everyone else's clothes. They noticed compared to everyone else in the room, it looked like they were wearing pyjamas and they were out of place.

"I think I can help you out with that" Iris cut in "I have some old clothes at my Dad's house that I think might fit Katara. As for Aang, Barry might have some stuff in the attic."

"That's perfect. Can you guys go do that now?" Caitlin asked. Iris nodded.

"Alrighty then, my work here is done. If you guys need anything call me, and Barry, actually call me." Felicity said as she walked out the door. Barry laughed.

Without asking anyone, Barry picked up Katara and flashed her to their house. About ten seconds later he did the same with Aang. When he got back Iris gave him that look. Barry laughed before doing the same with Iris.

"While they're doing that, I'm sure that we could all benefit from a brief resting period." Dr Stein said to everyone. They nodded in response. Caitlin and Ronnie left together , while Cisco, Joe and Dr Stein went to Big Belly Burger.

When Iris arrived she practically dragged Katara up the stairs to her room. Barry laughed and signaled Aang to follow him, and the two walked up the stairs to the attic.

Katara was overwhelmed with all the clothes that Iris had, she didn't understand why someone would need this many clothes in so many different styles when back home you had the same kind for at least two years depending on how you grew.

"But this isn't home. This isn't our world." Katara thought to herself. She tried on almost everything while Iris chose outfits and gave her things that could work with more than one pattern.

"Thank you so much for your help. Aang and I really appreciate it." Katara told Iris. Iris stopped ransacking her closet and looked at her.

"It's no problem really. I just hope that we'll be able to get you home." Iris replied.

"Yeah" Katara said sadly. "Home."


	6. Chapter 6

**Hey guys! Merry Christmas! I promised yesterday that I would post the second half of yesterday's chapter today. I'll try and update more often but I get really busy during school. Please leave a review! Happy Holidays!**

I do not own the Flash or Avatar the last Airbender

"Here you go man." Barry said while handing Aang some clothes. Aang took them and went behind the old screen to put some on.

"Thanks" Aang said to Barry. The polar opposite to Iris, Barry had marched up to the attic and dug out whatever he could find that looked it would fit Aang. After five minutes Aang had everything he would need for a month.

"So, if you don't mind me asking, what's your home like?" Barry asked. Aang perked up a little.

"It's beautiful. We have what's called bending. Like what Katara has, she's shown you right?" Aang asked, Barry nodded. " She has water bending. There are four different types of bending because there are four different nations ;Water Tribes, Earth Kingdom, Fire Nation and Air Nomads. Normal people can only bend one element but the Avatar can bend all four."

"That's cool, have you ever met the Avatar before? You're an Air Nomad right?" Barry asked.

" Yes I'm an Air Nomad, I'm the last one and as for the Avatar you could say that I've met him." Aang replied. He had a kind if sad look on his face.

" Why are you the last one?" Barry asked Aang. He was obviously confused and Aang wondered whether or not to tell him about how he was the Avatar and the 100 year war. _I can trust him_ Aang thought _He is a good person, I can trust him._

" There is something called the Avatar cycle. The Avatar is thousands of years old and is reborn in the next elements nation, for example if the previous Avatar was Water Tribe the next incarnation would be from the Earth Kingdom. You with me so far?" Aang asked. Barry nodded and set his head on his arm clearly interested.

"Over one hundred years ago the Avatar was Fire Nation. He died and was reborn into the Air Nomads. That's where I come in. At that time, the Fire Nation was ruled by Fire Lord Sozin. He was very close minded and started a war with the other three nations to try and take them over."

" The Avatar's job is to maintain peace within the world and to maintain peace between the physical world and the spirit world. Obviously keeping a major war from practically destroying the world is a job for the Avatar. But he disappeared."

Barry sat for a minute. Aang watched him to see if he put two and two together. It didn't seem like it yet, he would have to give some more clues.

"Ok, so then what happened?" Barry asked, he was like a five year old hearing a fairy tale.

"The Fire Nation killed all of the Air Nomads, every last one. They were ruthless. They tried to do the same with the other four nations but they were stronger and more prepared. The war went on for one hundred years until the Avatar came back. He stopped the fire lord and our world is now recovering and is at peace." Aang said. Barry looked confused.

"Ok, but if the Fire Nation killed all the Air Nomads, how did you survive?" Barry asked. Aang shrugged.

 **Avatarverse:**

Uncle was quiet, which was rare and not a very good sign. Five minutes ago, his nephew, his girlfriend and all of their friends who were supposed to be on Ember Island came riding up on Appa with horrible news that the Avatar and his girlfriend had been sucked into a hole in the sky and haven't been seen since.

"Do you think you can help us?" Sokka asked. He and Uncle talked for a minute while Toph, Suki, Zuko and Mai had a quick chat.

"Anyone else noticing how desperate Snoozles sounds right now?" Toph asked, she knew how bad the situation was but even she knew that Sokka was overreacting just a little bit.

"Toph, his sister and best friend got sucked into a hole in the sky. It's only natural for him to feel this way. Even if he is extra grumpy." Suki said, trying to defend him a little but failing slightly.

"I know that, I'm worried about Twinkle Toes and Sugar Queen too but we need to be rational about this." Toph said.

"Coming from you, that's pretty serious." Zuko said. Mai nodded.

Sokka and Uncle turned to the rest of the group.

"We're going to organize an emergency meeting with the leaders of the Four Nations." Sokka said.


End file.
